If you create a scripted field with a Type of "string", a Format of "url" and the Script of: 'http://testwebsite.com/' + doc['geo.dest'].value + '/' + doc['geo.src'].value it'll create a URL using two fields from the document

@softec17 that warning/error that you're seeing in Discover doesn't appear to be related to the Scripted Field, but because a shard failed when executing the query. If you remove the scripted field, do you still see the warning/error?